Stay warm out there

Yesterday and today, we woke up to a temperature of -3°F. Three below zero is cold. Yet I bundle up and go outside to fill the bird feeder and the cat dishes and put fresh water down (that's not a solid block of ice), and the animals come. Squirrels, rabbits, tons of birds, the two feral cats, even a few deer came by last night.
I can't stand to be out in it for more than 15 minutes at a time, but all these creatures LIVE in this weather. How do they do that?

This is how:
There is a wide array of morphological, physiological, and behavioral adaptations for winter survival.  A few examples are provided below, but investigations into the lives of active winter animals will reveal many combinations of survival strategies.

• Bergmann's Rule states that northern species of a particular genus or similar class of birds or mammals tend to be larger in size, although this is not always true.  Larger body size means a higher body mass-to-surface area ratio.  It's easier to retain heat.  Polar bears are larger than tropical bears.  White-tailed deer in Michigan dress out at higher weights than their counterparts in Texas or Florida.  
• Body appendages tend to get smaller in the north, as a heat conservation measure.  Snowshoe hares have smaller ears that cottontail rabbits.   Mammalian legs and snouts are frequently shorter and stouter.
• Specialized fat, called brown fat, is produced during the food-rich seasons and expended during cold seasons.  This is also the kind of fat that most hibernators use for arousal and many migrators use for fuel.
• Various "heat exchange" mechanisms can be found in animal circulatory systems that reduce heat loss to body extremities.
• Certain fish and herptiles produce chemicals within and between cell walls that can lower their freezing temperature a few degrees.  In sheltered environmental niches, these few degrees can mean the difference between life and death.
• Some mammals, such as flying squirrels and small rodents, will occupy collective dens to conserve body heat, even though some species are non-colonial during the warm season.  This is part of the reason that some species of snakes will do the same thing.
• Food preferences change with the season.  Some browsers, such as white-tailed deer, have changes in digestive enzymes to cope with the different food sources.  This is one of the reasons why biologists argue against winter deer feeding.  If not done correctly, a deer can starve to death with a belly full of corn.
• Ruffed grouse "snow roost" during periods of extreme cold.  Snow provides a very effective barrier against severe cold.    They will rest under the snow until the severe weather passes.  Folks who snowshoe or cross-country ski too close to these snow roosts are often caught off-guard when a grouse explodes out of the snow.  Large piles of grouse droppings are spring-time indicators of how severe the winter was. 
• Aquatic mammals, such as otter and mink, grow thick layers of insulating fat and have specialized fur.  Similarly, ducks, geese, and swans have feathers and oil glands that keep water away from the skin.  Some have efficient circulatory heat exchangers between the body and the feet.  It's usually not the cold that causes waterfowl to migrate.  It's more a matter of food shortages. 
• Birds and mammals undergo seasonal changes in feathers and pelage.  Trappers know that winter pelts are the highest quality because they are thicker and have different kinds of hair.
• Muskrats and beaver construct shelters, partly for protection from severe weather.  A number of animals dig burrows, such as groundhogs, foxes, chipmunks, and moles. 
• Many species of birds can adjust their internal body temperature downward to reduce the temperature gradient with environmental temperatures, thus reducing heat loss.  They also tend to shiver a lot to maintain body temperatures.